http://www.voipbuster.com is even better! You can try it for free (up to one minute for each call) and if you just deposit 5 EUR (+ VAT) (using Credit Card, Paypal, ...) you can call regular phones in these countries for free:

Austria

Denmark

Finland

Germany

Greece

Ireland

Luxembourg

Netherlands

Norway

Portugal

Spain

Sweden

Switzerland

United Kingdom

United States

I love it! Quality is great!

It's even compatible to the SIP standard (can't give you details about that though since I use the VoipBuster program itself). Uses about 10-15 MB RAM, which is ok.